中央研究地球科學研究所

My research activities address the structure, dynamics, and composition of planetary interiors, and more specifically of the mantles of rocky planets, including our Earth, and of the ice layers of icy moons and dwarf planets, such as Europa, Titan, and Pluto. These activities imply a multidisciplinary approach that combines numerical simulation of convection, which is thought to be the main mode of heat and mass transfer within planetary bodies, mineral physics data, which gives information on the physical properties of planetary mantles’ materials, and of course geophysical observations when they are available. Because convection in planetary mantles may be affected by many different parameters, a central task of my research is to better understand the individual roles played by each of these parameters, by performing numerical simulations with different levels of complexities. Important parameters include the mode of heating (i.e., the source of energy driving convection), the rheology (the way rock or ice are deforming), and the presence of different chemical components with different densities. In the case of the Earth, different chemical components are likely present in the deep mantle, and may play a key role in explaining today’s structure and dynamics. The thermo-chemical structures predicted by simulations can be tested against existing observations, mainly from seismology, to identify the best possible model. Other observables that may provide hints on the structure of the deep mantle include the topography of the boundary separating the mantle and the core, and seismic attenuation. In the case of icy moons, models of convection may be used to reconstruct the thermal histories and radial structures of these bodies, which can be, again, tested against available data.

Stagnant lid convection in spherical geometry.

I conducted experiments of thermal convection in 3D-Cartesian (Deschamps and Lin, 2014) and 3D spherical (Yao et al., 2014) geometries, showing substantial differences compared to 2D-Cartesian experiments, which are usually used in to model the evolution of icy bodies. These experiments showed that the curvature of the shell alters the properties of stagnant lid convection, the stagnant lid being attenuated in shells with larger curvatures, and the heat transfer being more efficient.

Heat flux at the core-mantle boundary and core dynamics (with Dr. Hagay Amit and Dr. Gaël Choblet, University of Nantes).

We estimated the heat flux pattern from models of probabilistic tomography, and investigated the effect of this heat flux pattern on geodynamo models. The geodynamo models we obtained have more low-latitudes convective and magnetic activity than corresponding models obtained with conventional tomography, and they recover better the observed latitudinal distribution of the CMB geomagnetic flux (Amit et al., 2015). We started a new collaboration that aims at mapping CMB heat flux by combining models of convection and seismic tomography maps.

Lower mantle electrical conductivity.

I modelled lower mantle electrical conductivity from appropriate mineral physics data and 3D thermo-chemical models (Deschamps, 2015). In collaboration with Dr. Amir Khan (ETH Zurich), I have further calculated the geomagnetic responses (C-responses) associated with different conductivity structures corresponding to purely thermal and thermo-chemical models, and showed that these C-responses differ at long periods (Deschamps and Khan, 2016).

Post-perovskite patches within reservoirs of dense material (with Dr. Yang Li, now at IGGCAS, Beijing).

We calculated models of thermo-chemical convection showing that if the temperature at the core-mantle boundary (CMB) is not too high and not too low, typically around 3500 K, small patches of post-perovskite (pPv) may be present within the reservoirs of dense, primordial material. If these reservoirs are enriched in iron, pPv patches may provide an explanation for the ultralow-velocity zones (Li et al., 2016).

Shear-wave velocity and seismic attenuation (with Dr. Kensuke Konishi, IES, and Dr. Nobuaki Fuji, IPG Paris).

We inverted seismic waveform data for radial profiles of shear-wave velocity and seismic attenuation at the Eastern tip of the Pacific low-shear-wave velocity province sampling, in particular, the Caroline plume (Konishi et al., 2017). These profiles suggest that the Caroline plume entrain small amounts of dense material from the Pacific LLSVPs. We are now working on the interpretation of these results in terms of thermo-chemical structure, and on 3D models of attenuation and shear-wave velocity.

Dynamics of Sputnik Planitia (with Dr. Kenny Vilella, IES).

Sputnik Planitia is a nitrogen ice glacier at the surface of Pluto, showing polygonal patterns characteristic of thermal convection at its surface. Our calculations indicate that Sputnik Planum may be animated by thermal convection driven by variations of surface temperature due to long-term variations of Pluto’s orbital parameters (Vilella and Deschamps, 2017). These variations trigger long successive periods of heating and cooling of the glacier. Dynamically, cooling is equivalent to a source of internal heating, and is thus able to power convection.

Lower mantle thermal conductivity (with Dr. Wen-Pin Hsieh, IES).

Dr. Wen-Pin Hsieh, assistant researcher at the IES, recently measured thermal conductivity of magnesium and iron-rich bridgmanite and ferro-periclase up to pressures corresponding to the bottom of the mantle. Using these data in combination with thermo-chemical models of lower mantle, I modeled radial and azimuthal variations of conductivity in the lower mantle, together with their implications for heat flux at the core mantle boundary and for the evolution of ultra-low seismic velocity zones (Hsieh et al., 2017; Hsieh et al., submitted).

Dynamic topography at core-mantle boundary (with Prof. Paul Tackley, ETH Zurich, and Prof. Yves Rogister, EOST Strasbourg).

I have calculated CMB dynamic topography induced by mantle flow for different models of mantle convection. Thermo-chemical reservoirs induce depressions in the CMB topography, with depth depending on the chemical and viscosity contrasts. Plume clusters, on the contrary, induce CMB elevation. Our results further suggest that long-wavelength of CMB topography, if they can be observed by seismology, provide important clue on the nature, thermal or thermo-chemical, of low shear-wave velocity provinces (LLVSPs) observed by global tomographic images (Deschamps et al., in revision).